1. Field of the Invention
The present invention relates to a signal processing circuit such as compressors, limiters, and expanders where the level of an output signal is controllably varied in accordance with the level of an input signal.
2. Prior art
Listening characteristics of conventional audio equipment such as CD players, digital audio tape recorders, cassette tape players, and radio tuners are greatly different according to sound sources (classical music, jazz, popular music, conversation, etc.) and listening environments (in a room, on the train, in a car).
It is difficult to hear quieter parts of a tune being played in a noisy environment such as cars and trains. Increasing the volume of a reproduced signal to an audible level may cause sounds which are too loud for relatively louder parts of the tune. On the other hand, the volume cannot be increased too much when listening in a very quiet place, making it difficult to listen to the details of quieter parts.
In order to eliminate the aforementioned problems, ordinary audio equipment is provided with a compressor where the output signal is compressed in dynamic range, a limiter where the maximum level of the output is limited, or an expander where the dynamic range of the output signal is expanded.
FIG. 5 shows a compressor used in a conventional audio system. The compressor has a variable gain amplifier 52 placed in a signal line 51, an input level detector 53 for detecting the level of an input signal, and a gain setting section 54 having a previously determined input vs output table as shown in FIG. 6.
The input level detector 53 detects the level of the input signal to cause the table in FIG. 6 to output a corresponding gain of the amplifier 52, so that the gain of amplifier 52 is controllably varied to effect compression of signal with the input-output characteristic shown in FIG. 6. In this manner, the dynamic range of the output signal is compressed such that the volume of quieter parts of a tune is increased while the louder parts being decreased. The conventional audio system uses only one previously determined table of input-output characteristics for compressing dynamic range and therefore suffers from the following drawbacks.
Compressing the dynamic range with the characteristic shown in FIG. 6 tends to make the quieter parts of a tune more noticeable than the louder parts. Accordingly, when the overall volume is increased, some input-output characteristics may emphasize not only quieter parts of a tune but also hiss noise and background noise, and may cause clipping of louder parts at subsequent amplifiers and speakers. When the overall volume is decreased, some input-output characteristics may cause the quieter parts to be masked in the background noise. Thus, the effect of compression is not always the same but different depending on the volume settings of an audio system. The use of a single input-output characteristic table is not sufficient for desired overall compression effect on all possible settings of volume. The same is true of limiters and expanders.